Frame Fabrication

From The Scratch-Built Hot Rod

Chapter 2: Frame Fabrication

Cutting the frame sections

With your frame sketch in hand, you can head out to the shop to begin turning your dream into reality. What this chapter will be showing is the fabrication of a "ladder frame", which takes its name from its straight rails on each side, and a series of steps or crossmembers to keep those side rails stable and square. This frame style is a staple of hot rods going back to the early use of Model T frames for souped-up bombs from the 40's and 50's.

Using the scaled measurements on your plan, you first cut the side rails and the two elements of the rear kick-up, commonly referred to as a "Z". I am using 2" x 3" 1/8" wall rectangular steel tubing for the frame shown here. Tubing size will depend somewhat on your choice of engine, the horsepower you intend to create, and the amount of "X" member or "K" member support you intend to include in the frame design. There are instances of builders using smaller 1 1/2" x 3" tubing for lower horsepower engines and 2"x4" or even larger tubing for higher horsepower applications.

A 14" chop saw is used to cut the tubing pieces, which keeps the ends fairly square and true. The basic tubing pieces for the frame rails are shown in Photos 2-1 and 2-2.

Creating a front frame horn

Although it isn't absolutely necessary, a more traditional look for the frame rails can be achieved by creating what is known as frame horns at the front. This frame shape is found in many stock frames from the 20's and 30's and makes the home-built chassis stand out from typical square-cut tubing corners.

To create the front frame horn, first draw a pattern on heavy paper stock (Photo 2-3). You'll have to experiment a bit to see what looks proportionally correct for the tubing size you use. Then, cut out the pattern and transfer the outline to both sides of your tubing (Photo 2-4).

Then, cut the tubing. A 4 1/2" grinder with a thin-blade cutting wheel works well for this task, but other cutting tools, like a metal-bladed saber saw, can also be used. Your frame horn should look like Photo 2-5 after you slice it up.

Use clamps and some hammer work to bend the top piece of metal over the curved side sections, and then tack weld it in place. You will also need to cut a small section of sheet metal to piece in at the front tip of the horn, to round it off and fill in the gap shown in Photo 2-6. Photos 2-7 and 2-8 show the frame horn after welding, grinding smooth and applying a quick coat of primer to protect the metal from rusting as fabrication proceeds.
Speaking of grinding the welds, you will note throughout the chassis fabrication the welds are ground as we go. That is not mandatory. Some builders, who are excellent welders, do not grind and smooth their welds at all, leaving them as testament to their craft and workmanship. Or, they simply prefer the look of a welded seam versus a smooth seam. This is totally up to the builder's personal tastes.

However, if you leave your welds raw, it is highly recommend you wire brush them immediately and then apply a quick primer to prevent oxidation. And if you do intend to smooth all your weld joints, it is best to do your grinding immediately. There is nothing more dreaded than staring at a week's worth of weld grinding. By doing your grinding and smoothing after each small section of welding, you break that loathsome job into lots of little pieces, making the task much less daunting.

Assembling the side rails

You are now ready to weld together the side rails. Be aware, however, that frame rails are subjected to a good deal of stress, both from horsepower loading and from road conditions. You don't want your frame cracking or actually breaking apart due to these stresses. To make your frame more stable, you should go beyond simply butt welding the frame pieces together. This can be done through the use of hidden gussets to reinforce the critical joints of your frame.

The hidden gussets for any given frame configuration will be different. These are the gussets used for the "Z" in the frame rails of this project. They are cut from 3/16" flat stock (Photo 2-9).

To avoid having these gussets slapped on the outside of the frame and looking ugly, you can hide them on the inside of frame tubing. Photo 2-10 shows a gusset being welding to the inside of the frame rail tubing using a stick welder. This is one time when a stick welder trumps a MIG. The MIG nozzle simply won't fit in the confines of the tube well enough to get a good weld.

With one end of the gusset welded in place to the main section of side rail, you next drill a series of 1/2" holes in the "kick-up" section of the frame rail as shown in Photo 2-11. Try to spread the holes fairly evenly over what will eventually be the surface of the gusset.

Continue by welding a gusset inside the other side of the frame rail and drilling holes on the opposite side of the kick-up section. The two frame sections can then be slipped together manually as shown in Photo 2-12. To better show how the rail sections are slipped together, this facsimile photo was created in Photoshop. But it will demonstrate how the hidden gusset can now be "plug welded" to the kick-up frame section by welding around the joint created between the edge of each hole and the gusset directly underneath it. The rest of the hole is then filled with weld and the area ground smooth and flat. Together with a good butt weld where the two sections meet, this should provide a strong and long-lasting joint.

Before doing those plug welds, however, make sure the rail sections are square and true. This can be done by slipping together the three sections that have been prepared for gussets: the main side rail, the kick-up section and the flat rear section at the top of the "Z". Then take measurements to ensure the total frame length matches your sketch length, the top frame section is absolutely parallel with the main side section, and that the angle of the kick-up matches your sketch.

With one side rail completed, the second rail is slipped together in the same manner, and then clamped to the completed first rail as shown in Photo 2-13. This ensures that when the second rail is tack welded, it will exactly match the first rail. And if your first rail was square and true, your second will be also.

Photo 2-13 With one frame rail welded, clamp the second rail to it, so that they will match when the second is welded. Photo attribution

Those of you with a sharp eye may be wondering about the "assembly table" which is shown in these photos supporting the frame components during assembly and welding. Most professional shops use a rigid jig or table, often constructed of steel I-beams or H-beams to ensure that the frame does not warp during welding. If you can find the appropriate materials to construct such a jig, and they are within your budget, by all means build a flat jig/welding table.

What is being used here is a 1 1/2" thick oak table top mounted on a 2x4 frame. Needless to say, it does little to prevent potential warping of the frame during welding. However, this warping has been found in the past to be a negligible, 1/8" to 3/16", corner-to-corner variation. This variance can be easily overcome by making some slight adjustments during assembly of the suspension system. Clearly, a rigid jig is the preferred alternative. But many hobbyists have built perfectly acceptable frames without one.

Attaching the crossmembers

Begin by clamping your side rails together as shown in Photo 2-14, so that you scribe matching lines which will be used to position your crossmembers where you want them. With the rails clamped, you may also want to scribe or permanently mark a reference point on each rail, top and bottom, with a punch. For this project, marks were scribed 4' from the end of the frame. This mark is used later as a reference point for installing the transmission mount, the front suspension components and other mounts and brackets.

Photo 2-14 Clamp the rails together and mark position of crossmembers. Photo attribution

Almost all ladder-type frames will consist of a crossmember at the very rear of your frame, another crossmember at or near the top of the frame "Z", and one near the front of the frame, but not encroaching on the curved area of the frame horn. There will also be a crossmember which normally doubles as a transmission mount. The transmission crossmember and a crossmember at the top of the "Z" will be cut and installed a little later in the process. For now, only the frontmost and rearmost crossmembers are welded to the chassis.
Cut the crossmembers to the correct length based on your frame sketch. For this project, the members are 28" long. This will set the outside width of the frame at 32" as required by our I-beam front suspension. The three members are shown in Photo 2-15. Note that the front crossmember has two large holes drilled in the bottom. This will be explained in detail in the next chapter, but they are for securing the mounting apparatus for the quarter elliptical front spring. It is much easier to drill these holes now with the drill press than wait until later when they would have to be drilled with a hand drill. Also, the third crossmember shown in this photo will be used only as a "dummy" at this juncture to help keep the frame square. It will be installed later at the top of the "Z".

The side rails are positioned on the assembly table and the crossmembers set in place (Photo 2-16). As the components are positioned and set square to one another, they are clamped solidly to the table and to each other. Note that the rear crossmember shown in this photo (farthest from the camera) is a dummy being used only for the purpose of squaring the frame. Since the transmission crossmember is not being installed at this time, the temporary crossmember is needed to help keep the lower side rails in their correct position. The dummy crossmember will not be welded in place.

You will need to recheck the squareness of all the components a number of times and make slight adjustments until they are all correct.

Next, go to the rear of the frame (Photo 2-17) and clamp the rear crossmember in place, again checking to ensure everything has remained square. Then move between the front and back of the frame, tack welding those two crossmembers in place while you continue to check that things have remained square. You can then proceed to final welding of the front and rear crossmembers.

Photo 2-18 An end plug being installed. Note the "handle" used while tacking. Photo attribution

As you begin to weld your frame pieces together, you will quickly discover that you have unsightly open-ended tubes at every corner (that is, unless you miter each corner at a 45-degree angle). These openings need to be sealed off by welding on an end cap or plug cut from 1/8" or 1/16" thick flat stock. Keep in mind that these end plugs are not just to give your frame a finished look. The plugs provide lateral strength to the rectangular tubing to prevent "racking" or twisting of the tube. Think of a cardboard box, which is easy to collapse if the top and bottom flaps are left unfolded. But fold the flaps together and run a piece of tape down the joint and you have a much more rigid structure. Your end plugs serve the same purpose as those box flaps.
To keep these plugs in place while you tack them in, a temporary "handle" can be made using duct tape (Photo 2-18). You can also tack weld a short length of wire to the end cap for the same purpose and then grind it off later. Just be aware, without some way of holding onto the plug during assembly, it is very easy to jar it out of position and end up with a cockeyed plug, not to mention a lot of frustration.

Photo 2-19 shows the front crossmember welded in place and being ground smooth while Photo 2-20 shows one of the end plugs being ground smooth after welding. The completed perimeter frame is shown in Photos 2-21 and 2-22.